An American study has found that partners in happy marriages have lower blood pressure than single people. Blood pressure was monitored over a 24 hour period in 204 married and 99 single men and women. High marital quality was found to be associated with lower blood pressure, lower stress levels, less depression and higher satisfaction with life generally.
Single individuals however, had lower blood pressure than those in unhappy marriages. Having a supportive network of friends did not appear to moderate the effects of being single or unhappily married.
(Is There Something Unique About Marriage? The Relative Impact of Marital Status, Relationship Quality, and Network Social Support on Ambulatory Blood Pressure and Mental Health. Annals of Behavioral Medicine, April 2008.)